While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, DUV and EUV lithography processes are thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using an ArF excimer laser as the light source is requisite to the micropatterning technique capable of achieving a feature size of 0.13 μm or less.
The ArF lithography started partial use from the fabrication of 130-nm node devices and became the main lithography since 90-nm node devices. Although lithography using F2 laser (157 nm) was initially thought promising as the next lithography for 45-nm node devices, its development was retarded by several problems. A highlight was suddenly placed on the ArF immersion lithography that introduces a liquid having a higher refractive index than air (e.g., water, ethylene glycol, glycerol) between the projection lens and the wafer, allowing the projection lens to be designed to a numerical aperture (NA) of 1.0 or higher and achieving a higher resolution. The ArF immersion lithography is now implemented on the commercial stage. As the exposure technology following the ArF lithography, EB and EUV lithography processes are regarded promising.
As the circuit line width is reduced by the recent rapid advance of technology, the degradation of contrast by acid diffusion becomes more serious for the resist material. This is because the pattern feature size is approaching the diffusion length of acid. Acid diffusion leads to degradations of mask fidelity and pattern rectangularity and non-uniformity of a fine line pattern, i.e., line width roughness (LWR). Accordingly, to gain more benefits from a reduction of exposure light wavelength and an increase of lens NA, an increase in dissolution contrast and suppression of acid diffusion are required more than in the prior art resist materials.
One approach to overcome these problems is to bind a PAG in a polymer. For instance, aiming to improve sensitivity, Patent Document 1 proposes a polymer using an acryloyloxyphenyldiphenylsulfonium salt as a monomer. Patent Document 2 proposes to incorporate the monomer into a polyhydroxystyrene resin for improving the LWR of this base resin. However, since the sulfonium salt is bound at its cation side to the polymer, the sulfonic acid generated therefrom upon exposure to high-energy radiation is equivalent to the sulfonic acids generated by conventional PAGs. These proposals are thus unsatisfactory to overcome the outstanding problems. Also, aiming to improve sensitivity and resist pattern profile, Patent Document 3 discloses sulfonium salts having an anion side incorporated into the polymer backbone such as polystyrenesulfonic acid. The acids generated therefrom are arenesulfonic and alkylsulfonic acid derivatives which have too low an acid strength to sever acid labile groups, especially acid labile groups in acrylate-derived base resins. The acrylate resins are commonly used not only in the ArF chemically amplified lithography offering a fine feature size, but also in the EB and EUV lithography processes. Also a variety of anion-bound resins capable of generating an acid having high acid strength have been developed. Patent Document 4 discloses a polymer having a difluoroethanesulfonic acid anion in the backbone. Patent Documents 5 and 6 disclose a polymerizable sulfonium salt having a partially fluorinated sulfonic acid anion and a resin obtained therefrom. Acid diffusion is suppressed by incorporating a strong acid-generating anion in the backbone of a base resin. Thus, some improvements are made in resist properties including mask fidelity, pattern rectangularity and LWR.
The lithography techniques which are considered promising next to the ArF lithography include electron beam (EB) lithography and extreme ultraviolet (EUV) lithography. In these techniques, exposure must be done in vacuum or reduced pressure, which allows the sulfonic acid generated during exposure to volatilize, failing to form a satisfactory pattern profile. Such volatile sulfonic acids or volatile sulfonium cation decomposition products (e.g., phenylsulfides) induce so-called “outgassing,” causing damages to the exposure system.
Herein Y—H denotes a proton donor such as a polymer matrix, and Xa− is an anion. The photo-decomposed product is exemplary while more complex photo-decomposed products may form. Some approaches are taken to reduce the outgassing from acid generators, as described, for example, in Patent Document 7.